But, the study says, reactive oxygen species - small molecules that are a natural by-product of oxygen metabolism and which react to stress by increasing to such an extent that they can damage cell structures - and energetic imbalances may play an important role in the sperm alterations.

By DPA, [RxPG] Amsterdam, July 6 - Intensive professional cycling training damages sperm, according to a study presented at the annual conference of the European Society of Human Reproduction and Embryology in Amsterdam recently.

Professor Diana Vaamonde, affiliated with the Spanish University of Cordoba Medical School, found that high-intensity training by triathletes significantly diminishes the quality of their sperm.

Previous studies by Vaamonde's team had already shown that both high exercise intensity and high exercise volume might be detrimental to sperm quality.

In the new study, in which the research team thoroughly analysed the sperm quality of 15 triathletes with an average age of 33, the scientists found a direct correlation between the volume of training in each activity and sperm quality.

Of the three modalities, only cycling, the activity for which triathletes undertake the most training, showed a clear correlation with sperm quality. The more time and distance covered in cycling, the worse sperm quality became.

'We found a statistically adverse correlation between sperm morphology and the volume of cycling training undertaken per week,' Vaamonde said.

The scientists say that the reduced sperm quality is most likely caused by a number of factors. Irritation and compression caused by friction of the testes against the saddle or the localised heat produced by wearing tight clothing might explain part of the problem.

But, the study says, reactive oxygen species - small molecules that are a natural by-product of oxygen metabolism and which react to stress by increasing to such an extent that they can damage cell structures - and energetic imbalances may play an important role in the sperm alterations.

Vaamonde said it was necessary to develop 'protective measures' for professional sportsmen to prevent damage to their sperm quality, including freezing their sperm prior to intensive training programmes.